Treatment of paint fillers and pigments



Patented-Dec. 14, 1943 TREATMENT OF PAINT FILLERS AND PIGMENTS Howard W.

Dicaliie C n tation of N Hall, Seattle, Wash, assignor to The P ompauy,LosAngeles, Calif a cor- No Drawing. Application December 15, 1941,

Serial No. 423,074

9 Claims. (Cl. 106-293) vance, but it is equally desirableand sometimesThe object of theinvention is to decrease the water absorption ofcertain pigments used in the manufacture of water paints.

In the manufacture of paints having an aqueous vehicle, such for exampleas the cold-water casein paints, water absorption is an important andoften a controlling factorin selecting a pigment or filler. The Federalspecifications covering paints of this type are so generally followed inthe trade that paints not conforming to them are not considered to be ofthe first quality in commerce, and these specifications closely limitthe water content.

For example, Federal Standard Stock Catalogue, section IV, part 5('I'If-P-23, December 18, 1935) specifies a maximum water content of 35%in cold water paint pastes, and also specifies minimum percentages oftitanium dioxide and zinc sulfid, or of titanium dioxide alone. Itfurther specifices that the paint most dilute with one-half itsvolume ofwater (which may contain a small amount oi hydrated lime) to a paint ofgood brushing consistency.

This specification is so drawn as to admit the use of a large quantity(up to 4870+) of inert fillers such as china clay, magnesium silicates,asbestin and diatomaceous earth. Diatomaceous earth is a highlydesirable filler in paints of this type. having important advantagesover the other fillers named, but its use has been attended by a highwater absorption which has, in many cases, made it impossible to meetthe specification for maximum water without so increasing the viscosityof the diluted paste that proper brushing properties are lost. Thisdrawbackhas seriously impeded the use of diatomaceous earth for this p Ihave discovered that when a minute proportion of citric acid is added tothe grades of diatomaceous earth used for this purpose, the waterabsorption is reduced'to a most surprising degree.

. For example, the normal water absorption of one well known brand ofdiatomaceous earth filler is about 220% of its dry weight, but by theaddition of $1 of 1% of citric acid the water absorption is reduced toabout 160% of the weight of the earth. The reason for his behavior isnot known to me, but I have noted that while the pH value of thediatomaceous earth filler is usually from 8.5 to 9.0, the dose of citricacid which gives the lowest water absorption is that which brings the pHof the earth to substantially 7.0. This dose and more convenientto addthe acid to a suspension of the filler in the aqueous vehicle. Forexample, in a paint having thefollowing formula I I I Pounds Vehicle (a20% casein solution) 125 Lithopo 160 Diatomaceous earth filler .50Asbestine v p 1 20 Water 24 Oil (for' emulsifying) it is desirable firstto earth with the vehicle, producing a thickly viscous suspension, thenstir in solution of citric acid until the viscosity has been reduced toa minimuma point which is readily observable after a littleexperiment-and then add the other ingredients of the paint in anypreferred order. This procedure avoids bringing the paste to anunworkable consistency at any time, which might happen if the earth andacid were added after the lithopone had been worked into the vehicle.

While I have not experimented with all of the acids which might be usedfor this purpose, I have found that tartaric acid and oxalic acid do notgive these desirable results and, so far as I am now-aware, the propertyof materially reducing thewater absorbing characteristic of pigments andfillers is unique to citric acid.

In applying citric acid -to other fillers and to paint pigments,variable results are obtained. With china clay there is no reduction inwater absorption while with the fillers having an alkaline reaction,such as the magnesium silicates, there is an appreciable reduction inwater absorption butless than in the case of diatomaceous earth. Thetreatment produces no result with the various lead pigments but ismoderately eflective with zinc sulfid and is still more eflective withtitanium dioxide, and particularly with either of these pigments whenintermixed, in the process of manufacture or otherwise, with even asmall proportion of diatomaceous earth.

may range from 0.01% to 0.05% of the weight of the earth.

In all cases in which the treatment is eflective the filler or pigmentis normally on the alkaline side of. the pH scale and the mostefl'ective dose is that which brings the mately 7.0.

I claim as my invention: 1. In the manufacture of paints having anaqueous vehicle: the step of incorporating in said paint a diatomaceousearth filler together pH value to approxi- 8 blend the diatomaceouswithsufiieierit citric acid to bring the pH value of said earth filler toapproximately 7.0.

2. In the manufacture of paints havin an aqueous vehicle: the step ofincorporating in said paint a diatomaceous earth filler together withcitric acid in quantity ranging from 0.01% to 0.05% ofthe dry weight ofsaid filler.

3. In the manufacture of paints having an aqueous vehicle: the step ofincorporating in said paint a pigment selected from the group consistingof zinc sulfid and titanium dioxide, a diatomaceous earth filler andcitric acid in quantity sufiicient to bring the pH value or the mixtureof pigment and filler to approximately 7.0. 4. A composition of matter:diatomaceous earth comminuted to a fineness adapting it to use as apaint filler plus a quantity of citric acid suilicient to bring the pHvalue of said filler to approximately 1.0. I

5. A composition of matter: diatomaceous earth comminuted to a finenessadapting it to use as a paint filler plus citric acid in quantityranging from 0.01% to 0.05% of the dry weight oi \said filler.

6. A composition of matter: diatomaceous earth comminuted to a finenessadapting it to use as a paint filler. together with a minute quantityofcitric acid, the water absorption or the composition being not over 160%by weight.

7. A composition of matter: a pigment selected from the group consistingof zinc sulfld and titanium dioxide; a minor proportion of finelycomrninuted diatomaceous earth, and suflicient citric acid to bring thepH value orthe composition to approximately 7.0.

' 8. A composition of matter: a zinc sulfld pigment; a minor proportionof finely comminuted diatomaceous earth, and suflicient citric acid tobring the pH value of the composition to approximately 7.0.

9. A composition of matter: a titanium dioxide pigment; .a minorproportion of finely comminuted diatomaceous earth, and sufilcientcitric acid to bring the pH value of the composition to approximately7.0.

HOWARD W. HALL.

